In a large pot or Dutch oven, melt the butter over medium heat.
Add the diced onion, carrots, potatoes, and mushrooms. Sauté for 5 to 7 minutes until the vegetables begin to soften.
Stir in the minced garlic and cook for 1 additional minute until fragrant.
Sprinkle the flour over the vegetable mixture and stir constantly for 2 minutes to create a roux.
Gradually pour in the chicken broth while stirring continuously to prevent any lumps from forming.
Add the diced chicken breasts and black pepper to the pot, then bring the soup to a gentle simmer.
Simmer uncovered for 15 to 20 minutes, or until the potatoes are fork-tender and the chicken is fully cooked.
Reduce the heat to low. Stir in the heavy cream, frozen peas, and frozen corn. Let the soup heat through for about 5 minutes without bringing it to a boil.
Garnish with freshly chopped parsley and serve warm.